Extraction of water vapor from gas streams is a critically important component of many process applications. For example, removal of water vapor is essential in corrosion prevention since many of the chemical reactions are catalyzed and accelerated by moisture. The prevention of the condensation in water treatment plants, ice rinks and refrigerated warehouses is another application that requires vapor extraction. The prevention of mold and/or fungal growth on, or within, archival materials, seeds, and foods is another important application in which water vapor extraction is required.
There are several ways to remove vapor, particularly water vapor, from a gas stream. One method is to cool the gas stream to condense the vapor. When air is cooled below its dewpoint temperature, moisture condenses on the nearest surface. The air is thus dehumidified by the process of cooling and condensation. Most commercial and residential air conditioning systems operate under this principle. Typically, a refrigeration system cools air, drains some of its moisture as condensate, and sends the drier air back to a space to be cooled. The system essentially pumps the heat from the dehumidified air to a different air system in another location, by using refrigerated gas to carry the heat.
Alternatively, one may present a desiccant to the gas stream, which pulls water vapor out of the gas stream through differences in vapor pressures. Desiccant dehumidifiers, instead of cooling the air to condense its moisture, attract moisture from the air by creating an area of low vapor pressure at the surface of the desiccant. The pressure exerted by the vapor in the air is higher, so the vapor molecules move from the air to the desiccant and the air is dehumidified.
Desiccant dehumidifiers make use of changing vapor pressures relative to dry air in a repeating cycle described by the simplified diagram of FIG. 1, showing desiccant moisture content on the x-axis plotted against desiccant surface vapor pressure on the y-axis. The family of curves in the Figure describes the behavior of the desiccant at different desiccant temperatures. The desiccant begins the cycle at Point 1. Its surface vapor pressure is low because it is dry and cool. As the desiccant picks up moisture from the surrounding air, the desiccant surface changes to the condition described by Point 2. Its vapor pressure is now equal to that of the surrounding air because the desiccant is moist and warm. At Point 2, the desiccant cannot collect more moisture because there is no vapor pressure difference between the desiccant surface and the vapor in the air.
Then the desiccant is taken out of the moist air, heated, and placed into a different air system. The desiccant surface vapor pressure is now very high, higher than the surrounding air, so moisture moves off the desiccant surface into the air to equalize the pressure differential. At Point 3, the desiccant is dry, but since it is also hot, its vapor pressure is still too high to collect moisture from the air. To restore its low vapor pressure, the desiccant is then cooled, returning it to Point 1 in the diagram and completing the cycle so that it can collect moisture again.
The desiccant cycle is driven by thermal energy. In the process of converting vapor in the gas to liquid in the desiccant, the vapor gives up its latent heat to the dehumidified air stream. The air exiting the desiccant is therefore warmed and must be cooled to return it to its original temperature. The desiccant also must be regenerated to remove the collected moisture and this is accomplished by driving off the liquid trapped in the desiccant through heating and re-evaporation. Normally, heated air is used for this process. The heat is provided by heat exchange with heated air warmed by an auxiliary heater. The moisture driven off from the desiccant is carried off in the regeneration air stream. The amount of heat needed for this process is substantial and regeneration of the desiccant must be done at elevated temperatures ranging from 150.degree. to 275.degree. F. (see FIG. 1).
As a result of this heating during regeneration, and the heat generated in going from the vapor to liquid state during the extraction of vapor, the desiccant itself is heated and a significant amount of heat is carried back over to the dehumidification process (Point 1 in FIG. 1). This carry-over heat is problematic in that the desiccant unit may not have cooled completely during its cooling cycle because residual heat is carried over into the new air stream. Heat carry-over greatly increases the amount of cooling needed for the conditioned air. Typical values of heat carry-over range from 60% to 200% of the latent load. Further, the air leaving the desiccant unit is both dry and hot and, in certain circumstances, must be cooled before being delivered to the point of use.
Recently, a gel-based dehumidifier has been proposed whose operation is based upon application of an electric field to a porous water-based polymer gel electrolyte. See, Japanese Patent Application 243516, Aug. 31, 1992, Fujitsu Inc. Application of an electric field between two electrodes to a water-based system leads to unwanted, and possibly dangerous, electrochemical reactions involving the water.